@ATX-dmh I've had similar stickers on a few of my packs in the past(also not from AU but my countries customs is very strict too). For me it seemed like they just put the sticker on it so they can fill a "has been inspected"-bag to fulfill a given quota or something like that. I've also had opened and resealed letters with green tape on it and after I've opened it and none of the reads were missing, I also got paranoid and didn't know what to think. But in the end, nothing happened whatsoever.
Also, even with seized packs(talking about "normal" quantities), being on some kind of blacklist has found to be bs and has been discussed many times on here and other places. So far, most US people which had packs seized just ignore the LL and go on ordering to the same address without any problems.
In other countries with more strict laws, most people I read of had to pay a fine or had to go to court and THEN had to pay a fine. But after that, many just go back and just continue ordering.
I'm still talking about normal quantities though! I guess you can imagine that things can become kinda difficult if a pack with 1k+ scheduled mags gets seized..
Also, if they'd really have a plan to do something in terms of prosecution, they'd need some kind of evidence, won't they? So, if it actually got opened and they saw the mags but didn't take them and let it go through, they would just give up that evidence which would also be the base for further steps. But they didn't, so in this moment, I wouldn't worry too much
Maybe wait a few days, look if something happens and try to calm down a bit. If you don't happen to get a letter or something else, you should be fine
All the best to you!
Edit: If you don't already do so, always make sure you're having a good opsec to prevent or soften the worst case.